This invention relates to secondary batteries and more particularly to secondary batteries having pressed or rolled powdered-type zinc anode in the uncharged and/or charged status.
Pure zinc-zinc oxide anodes in alkaline electrolytes generate hydrogen gas, with the gassing rate increasing with increasing temperature. The hydrogen gas forms on the cathodic sites of the anode by the decomposition of water. This hydrogen gas generation is particularly undesirable in sealed batteries where it can result in excessive pressure build up. Simultaneous to the hydrogen gas generation, active zinc at the anodic sites self-discharges into zinc hydroxide, zinc oxide, or mixtures of zinc hydroxide and zinc oxide, with a resulting loss in electrical capacity by the anodes. The overall self-discharge reactions are as follows:
Cathodic sites: 2H.sub.2 O+2e.fwdarw. H.sub.2 .uparw.+ 2OH.sup.- PA0 anodic sites: Zn+2OH.sup.- --2e.fwdarw.Zn(OH).sub.2 .revreaction.ZnO+H.sub.2 O PA0 overall: Zn+2H.sub.2 O.fwdarw.H.sub.2 .uparw.+Zn(OH).sub.2 .revreaction.ZnO+H.sub.2 O PA0 (a) from about 0.1 to about 5.0 weight percent of Tl.sub.2 O.sub.3, PA0 (b) from about 0.5 to about 10.0 weight percent of PbO, PA0 (c) from about 0.1 to about 5.0 weight percent of CdO, PA0 (d) a mixture of from about 0.1 to about 5.0 weight percent of Tl.sub.2 O.sub.3 and from about 0.5 to about 10.0 weight percent of PbO, PA0 (e) a mixture of from about 0.1 to about 5.0 weight percent of Tl.sub.2 O.sub.3 and from about 0.1 to about 5.0 weight percent of CdO, PA0 (f) a mixture of from about 0.1 to about 5.0 weight percent of Tl.sub.2 O.sub.3 and from about 0.1 to about 5.0 weight percent of SnO.sub.2, PA0 (g) a mixture of from about 0.1 to about 5.0 weight percent of Tl.sub.2 O.sub.3 and from about 0.1 to about 5.0 weight percent of In(OH).sub.3, PA0 (h) a mixture of from about 0.1 to about 0.5 weight percent of Tl.sub.2 O.sub.3 and from about 0.1 to about 5.0 weight percent of Ga.sub.2 O.sub.3, PA0 (i) a mixture of from about 0.5 to about 10.0 weight percent of PbO and from about 0.1 to about 5.0 weight percent of CdO, PA0 (j) a mixture of from about 0.5 to about 10.0 weight percent of PbO and from about 0.1 to about 5.0 weight percent of SnO.sub.2, PA0 (k) a mixture of from about 0.5 to about 10.0 weight percent of PbO and from about 0.1 to about 5.0 weight percent of In(OH).sub.3, and PA0 (l) a mixture of from about 0.5 to about 10.0 weight percent of PbO and from about 0.1 to about 5.0 weight percent of Ga.sub.2 O.sub.3. PA0 (a) from about 0.1 to about 5.0 weight percent of Tl.sub.2 O.sub.3 with from about 0.5 to about 10.0 weight percent of PbO, PA0 (b) from about 0.1 to about 5.0 weight percent of Tl.sub.2 O.sub.3 with from about 0.1 to about 5.0 weight percent of CdO, PA0 (c) from about 0.1 to about 5.0 weight percent of Tl.sub.2 O.sub.3 with from about 0.1 to about 5.0 weight percent of SnO.sub.2, PA0 (d) from about 0.1 to about 5.0 weight percent of Tl.sub.2 O.sub.3 with from about 0.1 to about 5.0 weight percent of In(OH).sub.3, and PA0 (e) from about 0.1 to about 5.0 weight percent of Tl.sub.2 O.sub.3 with from about 0.1 to about 5.0 weight percent of Ga.sub.2 O.sub.3, PA0 (f) from about 0.5 to about 10.0 weight percent of PbO with from about 0.1 to about 5.0 weight percent of CdO, PA0 (g) from about 0.5 to about 10.0 weight percent of PbO with from about 0.1 to about 5.0 weight percent of SnO.sub.2, PA0 (h) from about 0.5 to about 10.0 weight percent of PbO with from about 0.1 to about 5.0 weight percent of In(OH).sub.3, and PA0 (i) from about 0.5 to about 10.0 weight percent of PbO with from about 0.1 to about 5.0 weight percent of Ga.sub.2 O.sub.3 will maintain 95 percent or better of the capacity of the cell under the rapid cycling test procedure outlined in the examples. Preferred ranges for each of the additives when used in the binary mixtures are from 0.5 to 2.0 weight percent for Tl.sub.2 O.sub.3, from 1.0 to 5.0 weight percent for PbO, from 0.5 to 2.0 weight percent for CdO, from 0.5 to 2.0 weight percent for In(OH).sub.3, and from 0.5 to 2.0 weight percent for Ga.sub.2 O.sub.3. The weight percentages are based on the total weight of the binary additive mixtures plus the active zinc material in the electrode in the discharged state. Mixtures in the preferred ranges, in addition to suppressing hydrogen gassing and improving cell capacity on cycling, also showed no detectible amount of passivation during the low temperature-high discharge test (column 6 of the table) and only a small degree of self discharge during the charge stand test (column 5 of the table). The table as well as a description of the test procedures are located in the example section.
pure zinc-zinc oxide electrodes also lose electrical capacity on storage by the growth of zinc oxide crystals from active submicron-sized prismatic crystals to large, relatively inactive, orthorhombic crystals, up to 50 microns in diameter or greater. This process is also referred to as "aging."
A common prior art method for preventing the self discharge reaction of the alkaline zinc-zinc oxide anode is to raise the hydrogen overpotential of the anode by mixing or alloying the active zinc and its supporting grid (usually copper or silver structures) with 0.5 to 5.0 percent by weight of mercury or mercuric oxide. The mercury additive also minimizes passivation at high discharge rates and at low temperatures. To a small but appreciable extent mercury also reduces the "aging" rate of the zinc oxide crystals.
The major shortcoming of mercury (or mercuric oxide) as an additive to the alkaline zinc-zinc oxide anode is that it dissolves into the zinc lattice, forming an amalgamated surface with a reduced surface energy, or a lower activation overpotential, for the electrochemical exchange of ions during the charge/discharge reactions of the secondary battery. The amalgamated zinc surface practically eliminates the adatom diffusion step since the adatoms (surface adsorped species) can be incorporated at any point on an amalgamated surface. This phenomenon increases the rate of zinc corrosion and densification of the active zinc-zinc oxide material, processes which result in a fairly rapid loss in electrical capacity during the repetitive charge/discharge cycling of the battery. This capacity loss is accomplished by a rapid decrease in the real and the apparent surface areas of the anode, a process usually referred to as "shape change." For the same reasons, alloys of zinc and other additive metals produce cells which will show severe losses in capacity upon cycling.
U.S. Pat. No. 3,580,740 issued to Herbert I. James and U.S. Pat. No. 3,639,176 issued to George F. Nordblom and Herbert I. James disclose the use of mercuric sulfide, lead sulfide, or mixtures of mercuric sulfide and lead sulfide as hydrogen gas suppressant additives for zinc-zinc oxide anodes. Although these additives did not reduce the capacity of the battery as much as mercuric oxide did during cycling, they were much less effective in reducing the generation of the hydrogen gas.